Flexible roofing membranes are commonly formed of bitumen that has been modified by thermoplastic polymers such as block elastomers (SBS—Styrene Butadiene Styrene, SEBS—Styrene Ethylene Butadiene Styrene) or plastomers (Polyolefin such as PPA, EVA, PPI, APAO—Amorphous a polyolefin). The modification of bitumen results in a bitumen mixture having a softening point of about 120-130° C. and also being flexible at temperatures below 0° C.
Typically, membranes made from bitumen that has been modified are sensitive to UV light thereby causing increased rates of degrading of such bitumen mixture. In the past, this limitation of bitumen that has been modified has been addressed by adding metal particles and/or granules to the modified bitumen mixture and/or by adding metal particles and/or granules on the top surface of the bitumen mixture. In addition to UV light sensitivity, certain bitumens that have been modified are susceptible to increased rates of oxidation, thus causing the bitumen mixture to degrade and compromise the seal formed by the bitumen membrane. As such, many membranes that are formed by bitumen that has been modified require resealing every 15-20 years.
A modification of bitumen that has improved UV light sensitivity is disclosed in U.S. Pat. No. 7,374,816, which is incorporated herein by reference. The '816 patent discloses a bitumen base composition that includes 40-90 wt. % bitumen, 10-50 wt. % thermoplastic polyurethane, 0-10 wt. % aromatic oil, 0-50 wt. % filler, and 0-0.5 wt. % catalyst. The '816 patent discloses that the thermoplastic polyurethane is formed in situ in the bitumen base composition. Although the '816 patent discloses that the inclusion of aromatic oil is optional, the absence of such aromatic oil in the bitumen base composition will result in a very inflexible bitumen base composition. As such, a significant amount of naphthenic or aromatic oils must be added to the bitumen to maintain the flexibility of the bitumen base composition. Unfortunately, the use of naphthenic and aromatic oils can be environmentally unfriendly and can also be unpleasant to use. In addition, these oils, while acting like plasticizers to improve the cold temperature properties of the bitumen base composition, have a tendency over time to defuse out of the bitumen base composition and escape from the surface of the bitumen base composition, thus leaving the bitumen base composition in a more brittle condition than when the bitumen base composition was first applied.
In view of the current state of the art of bitumen coatings that have been modified, there is a need for an improved bitumen composition that resists UV degradation and maintains its flexibility.